The use of high strength fiber-reinforced composite material in the manufacture of aircraft and other lightweight structures has increased steadily since the introduction of such materials. Composite materials have a high strength to weight ratio and stiffness. These properties make composite materials attractive for use in the design of lightweight structures. One of the major drawbacks to using composite materials however, is the relatively high cost of production using these type of materials.
When a bonded composite assembly is fabricated it is "layed-up" in a bonding tool. Most composite assemblies consist of a skin (usually fiberglass, graphite, or aluminum) that sandwiches a core material that is also usually fiberglass, graphite, aluminum, or titanium. The core is generally hexagonal in shape although other core shapes can be implemented as well. There are several different variations of the composite assembly process in use, but all are eventually autoclave or oven cured in a bonding tool.
During the curing process a composite assembly is sealed inside a vacuum bag and then placed in an autoclave or oven, for curing at elevated pressures. To prevent damage to the composite assembly during the curing process, fairing bar are required to be used. The function of the fairing bars is two-fold: (1) to keep the vacuum bag from breaking due to bridging over voids or gaps along the edges, and (2) to prevent the composite core from being crushed in a "domino effect" due to the lateral forces from the pressurization. Composite materials are typically quite susceptible to collapse from lateral forces and thus need to be supported laterally in order to withstand the forces created by the elevated pressures of the curing process.
Fairing bars are typically positioned around the periphery of each composite assembly. Typically between 12 and 20 fairing bars are used per tool, although this number can vary widely depending upon the size and complexity of the composite assembly. The composite fabrication process requires that the fairing bars be removable. Thus, each fairing bar is configured to be bolted or otherwise hard fastened to the bonding tool, usually with a minimum of three bolts per fairing bar. Such bolts engage vacuum bushings inserted through the facesheet of the tool at each bolt location. Inserting such bushings typically requires that a computer controlled six axis mill be used to bore each hole, countersink the hole, and then backspotface the hole. After each hole is machined, an internally threaded vacuum bushing is inserted, sealed (usually with a silicone adhesive), and then checked for integrity.
The bonding tool is then released to production workers who bolt each fairing bar in place during the composite fabrication process. In order to hold each fairing bar in position bolts are used that are approximately 0.5 inches to 2.0 inches in length.
Prior to bolting the fairing bars in place, the bolts are coated using a releasing agent, such as Frekote.RTM.. This releasing agent is designed to prevent the composite resins, which flow freely in the composite assembly during curing, from permanently adhering the bolts to the fairing bars and the bonding tool and thus, the fairing bars to the bonding tool. Following each oven or autoclave cycle, these bolts are removed along with the fairing bars. At this stage bolts are often lost, requiring the acquisition of replacement bolts. In addition to the time required to obtain new bolts, further time is also required for each new bolt to be coated with a releasing agent.
For the manufacture of most parts, production workers must spend several hours locating bolts and securing the fairing bars to the bonding tool. This process is then subsequently repeated, since these fairing bars must be removed after every autoclave or oven cure cycle. In approximately 25% of any given run of composite parts, at least one of the bolts becomes bonded into the tool, thus locking the fairing bar to the bonding tool. This can require several further hours to remedy. Also, approximately 0.25% of all composite parts have to be scrapped due to loss of vacuum pressure during an autoclave cycle. This is directly attributable to vacuum bushings leaking, which occurs as a result of over torquing of the bushings and cracking of the bushings due to temperature and pressure cycling.
Previous attempts have been made to avoid the above mentioned problems and costly production time requirements. One such attempt to quicken the indexing and securing of fairing bars to a bonding tool, relied on "bullet nosed liners" to replace bolts. This technique basically mated short male pins with female bushing. This technique was unsuccessful however in that the pins had a tendency to fall out and/or break under heated, pressurized conditions. This pin/bushing method was also ineffective in holding a fairing bar to a vertical surface. Thus, a sound technique is still needed that can provide the benefits of securely bolting fairing bars to bonding tools without the inherent costly production time delays resulting therefrom.